Dry plate selenium rectifier



face.

Patented Jan. 3, 1950 DRY PLATE SELENIUM RECTIFIER Stanley 8. Fry, North Chicago, 111., assignor to Fansteel Metallurgical Corporation, North Chicago, 11]., a corporation of New York No Drawing. Application November-11, 1944, Serial No. 563,103

7 Claims. (01. 175-368) Thls invention relates to blocking layer devices such as selenium rectiflers and light sensitive devices, and more specifically to means for forming an artificial blocking layer on the selenium.

In the production of blocking layer devices having a semi-conductor layer, for example. selenium rectiflers, a rigid carrier electrode or supporting electrode is provided with a thin-coating or layer of selenium. The supporting electrode may be formed of nickel, nickel-plated iron, aluminum, magnesium, beryllium or other metals and alloys. A common practice in the production of selenium rectifiers includes grit blasting the iron disc or plate and electroplating the disc-with nickel. The purpose of the grit blasting is to present a roughened surface to the selenium and to thereby improve the adherence of the selenium to the carrier electrode. The disc is then thoroughly cleaned and a thin film of selenium is applied over the nickel layer.

The selenium film or layer may be formed by a variety of methods. The plate or disc may be heated to a temperature above the melting point of selenium, for example, to a temperature of from 230 to 250 C., and the selenium in stick 25 form may be rubbed across the heated plate in order to melt the selenium and form the desired film. Another method of application includes placing a measured quantity of powdered selenium or selenium in pellet form on the heated plate and flowing the melted selenium overthe sur- The melted selenium is usually spread over the heated plate by mechanical means, as with a glass rod. The selenium may also be deposited on the carrier electrode from a vapor phase. The vaporization method is commonly employed in depositing the selenium film upon the light metal carrierelectrodes. Various materials may be added to the selenium to increase its conductivity and otherwise impart desired characteristics and properties.

- The selenium is then transformed into its gray, crystalline state by heat treatment. The coated discs are generally stacked with mica, aluminum or other inert, smooth-surfaced discs or washers in contact with the selenium and between ad- Jacent plates, and the stack subjected to a moderate pressure. The stacks under pressure are then subiectedto a relatively low temperature,

2 tained at such temperature for from one hour to four or five hours. During this stage of the heat treatment the selenium softens so that the pressure produces a layer of selenium of relatively uniform thickness and having a smooth surface.

The stacks are withdrawn from the oven or heat treating furnace and the plates. are removed from the stack. The plates are then given a further heat treatment at a temperature approaching the melting point of selenium. This heat treatment is generally conducted at temperatures between about 200 C. and the melting point of selenium, for example, about 210" C., for a period of from fifteen minutes to several hours. During the combined heat treatment, a layer of selenium is produced having a smooth surface of more or less uniform thickness and the selenium is transformed from its amorphous, non-conducting form into its gray, crystalline, conducting form.

The smooth surface of the selenium film is then' treated to form an artificial blocking layer and a counter-electrode consisting of a relatively low fier plates consists of an electrical forming treat-' ment. This treatment consists of subjecting the plate to either an alternating or direct current until a high resistance is developed in the reverse direction. This step may consist of applying to the plate in the reverse direction a direct current voltage below about 15 volts or a pulsating direct current startingwith a low voltage of about 8 volts and gradually increasing the voltage to about 21 volts over a period of several hours.

In the use of an alternating current, it is necessary to include in the circuit a current limiting resistor because one-half cycle flows in the forward direction of the rectifier plate. The voltage may be about 20 volts and the period required for forming is generally greater than that required when a direct current is employed because only one-half cycle of the current flows in the reverse direction.

Light sensitive devices of the blocking layer class are produced in a similar manner. A supthat is, to a temperature below 150 0., and mainso portins e ode s provided with a thin mm or layer of selenium in its my. crystalline modification and an artificial blocking layer formed on the surface of the selenium. A second electrode is then applied over the treated surface of the selenium. This electrode may consist of a lightpermeablefilm of metal or a metal grid. The application of the light-permeable metal may be accomplished by any of the methods known to the art.

This invention is directed to a method of forming an artificial blocking layer on the film of selenium in the manufacture of blocking layer devices. This method is not limited to any specific form, type or composition of carrier electrode or counter-electrode. Any desired method may be employed in providing the carrier electrode with the film or layer of gray. crystalline. conducting selenium. The selenium layer may or may not contain added materials to impart desired characteristics and properties.

The principal object of this invention is to provide a method of forming an artificial blocking layer on the surface of the selenium layer.

Other objects and advantages of this invention will become apparent from the following description and claims.

The present invention contemplates subjecting the surface of the selenium layer in its gray. crystalline modification to hot cyanogen gas.

The specific details of the manufacturing procus of the element are dependent upon the type of unit being made and the class of service for which it is designed. For purposes of illustration, the production of selenium rectifier plates is described.

A carrier electrode consisting of a grit blasted, nickel-plated iron disc is provided with a layer or film of selenium in its my. crystalline form by any of the aforementioned methods. The selenium may contain added materials to improve the conductivity. as is well known to the art. The selenium surface is then exposed to cyanogen gas. at a temperature of from about 100 to about 500' c. The period of exposure of the selenium surface to the hot gas may vary from a few seconds to several minutes, I prefer to expose the selenium surface to the hot gas for from about five seconds to about fifteen seconds.

The plates may be exposed individually to the hot gas by manually or mechanically passing each plate through the gas. The plates may be placed upon an endless conveyor belt which passes through a chamber having the desired atmosphere. The gas may be forced through a conduit and blown across the selenium surface. v

The counter-electrode is subsequently applied as by spra na' a low melting point alloy over the treated selenium surface. The rectifier plate is finally subjected to any of the desired electrical forming processes known to the art.

4 orderofllomilliamperes. Platesmadeinaeoordance with the present invention from the same sample of selenium will pass a current of the order of 6 to 8 milliamperes.

Another common method for grading selenium rectifier plates is the determination of the rectification ratio at 1 volt and 3 volts. This ratio is the ratio between the current fiow in the reverse direction and the current flow in the forward direction upon the application of a direct current voltage in thereverse and forward directions. The rectification ratio for theoretically perfect rectifiers is' 0. Rectifier platu having an area of about 12 sq. cm.madeinaccordancewiththis invention have a rectification ratio at 1 volt and 3 volts of the order of one-quarter to one-half the rectification ratio of plates made from the same sample of selenium without the blocking layer treatment of this invention.

These improvements are representative of the advantages obtained by the practice of my invention. It is to be understood that the foregoing numerical values will difi'er with different applied voltages, with different samples and grades of selenium. with different sise rectifier plates and with various substances which may be added to the selenium to alter its conductivity.

I claim:

1. The method of producing blocking layer devices which comprises providing a supporting electrode with a layer of gray, crystalline seleni- Various methods are commonly employed in grading and in evaluating the quality of selenium rectifier plates. One of these methods includes measuring the current flow in the reverse or blocking direction upon the application of a direct current voltage to the plate in the blocking direction. For an ideal or theoretically perfect rectifier plate. no current would flow in the reverse direction.

Inthe case of rectifier plates having mm of about 12 sq. cm. and made without the blocking layer treatment of this invention, the current flow upon the application of a direct current voltage of 20 volts in the reverse direction is of the 78 trode. the surface of the selenium being treated um, subjecting the surface of the selenium layer to the action of hot cyanogen gas and applying a counter-electrode over the treated selenium surface.

2. The method of producing blocking layer devices which comprises applying a layer of selenium to a supporting electrode, transforming the selenium into its gray. crystalline form, subjecting the surface of the selenium layer to the action of hot cyanogen gas and applying a counterelectrode overthe treated selenium surface.

3. The method of producing blocking layer devices which comprises applying a layer of selmium to a supporting electrode, transforming the selenium into its gray. crystalline form, exposingthesurfaceoftheseleniumlayerforfrom about five seconds to about fifteen seconds to hot cyanogen gas and applying a counter-electrode over the treated selenium surfa ce.

4. The method of producing selenium rectifier plates which comprises providing a supporting electrode with a layer of gray, crystalline selenium, subjecting the surface of the selenium layer to the action of hot cyanogen gas, applying a lenium surface and subjecting the composite unit 7 to an electrical forming operation.

6. A blocking layer device a supporting electrode, a layer of selenium on the electrode, the surface of the selenium being treated with hot cyanogen gas and a counter-electrode on the treated selenium surface.

7. A selenium rectifier plate comprising a supporting electrode, a layer of selenium on the elec- 5 8 with hot cyanogen gas and a, counter-electrode mrr over the treated selenium surface. FOREIGN PA a STANLEY S. FRY. Number Country Date 611,728 Germany Apr. 6, 1935 REFERENCES CITED 5 OTHER REFERENCES fi gm rgerences are of record in the Fuels and Furnaces, March 1932, pages 197 n and 198, an article by Wm; J. Merten-Special UNITED STATES PATENTS Uses and Newly Discovered Features of Nitrided Number Name Date 10 Steels and the Nitriding Practice.

2,121,603 Lotz June 21, 1938 2,227,827 Dubar Jan. 7, 1941 2,362,545 Ellis et 8.1 NOV. 14, 1944 

1. THE METHOD OF PRODUCING BLOCKING LAYER DEVICES WHICH COMPRISES PROVIDING A SUPPORTING ELECTRODES WITH A LAYER OF GRAY, CRYSTALLINE SELENIUM, SUBJECTING THE SURFACE OF THE SELENIUM LAYER TO THE ACTION OF HOT CYANOGEN GAS AND APPLYING A COUNTER-ELECTRODE OVER THE TREATED SELENIUM SURFACE. 